#line 1 "a.cpp"
#include<iostream>
#include<algorithm>
#include<vector>
using namespace std;
#line 2 "/home/kotatsugame/library/datastructure/UF.cpp"
struct UF{
	int n;
	vector<int>parent,rank;
	UF(int n_=0):n(n_),parent(n_),rank(n_,1)
	{
		for(int i=0;i<n_;i++)parent[i]=i;
	}
	int find(int a){return parent[a]!=a?parent[a]=find(parent[a]):a;}
	bool same(int a,int b){return find(a)==find(b);}
	bool unite(int a,int b)
	{
		a=find(a),b=find(b);
		if(a==b)return false;
		if(rank[a]<rank[b])
		{
			parent[a]=b;
			rank[b]+=rank[a];
		}
		else
		{
			parent[b]=a;
			rank[a]+=rank[b];
		}
		return true;
	}
	int size(int a){return rank[find(a)];}
};
#line 6 "a.cpp"
int N,L;
string S[1000];
main()
{
	cin>>N>>L;
	for(int i=0;i<N;i++)cin>>S[i];
	vector<pair<int,pair<int,int> > >E;
	for(int i=0;i<N;i++)for(int j=i+1;j<N;j++)
	{
		int c=0;
		for(int k=0;k<L;k++)c+=S[i][k]!=S[j][k];
		E.push_back(make_pair(c,make_pair(i,j)));
	}
	sort(E.begin(),E.end());
	int ans=0;
	UF uf(N);
	for(pair<int,pair<int,int> >e:E)
	{
		if(uf.unite(e.second.first,e.second.second))ans+=e.first;
	}
	cout<<ans<<endl;
}